The invention relates to a method of producing an electrode-electrolyte unit with a catalytically active layer.
Electrochemically operating units consisting of electrode-electrolyte -electrode are provided for example for use in fuel cells, electrolysis cells, or cells for electro-organic syntheses. The electrodes are preferably porous throughout so that operating means such as air and hydrogen can pass through the electrodes. In many cases, the electrodes participating in the electrochemical reactions must be activated by suitable catalysts.
For fuel cells whose operating temperatures are 0.degree.-150.degree. C., ion conductive solid electrolyte membranes are used. The anodes for the hydrogen oxidation and the cathodes for the oxygen reduction are coated mostly with platinum, recently also with a platinum-ruthenium alloy. The principle of such a membrane fuel cell is known from the patent publication "K. Kordesch, Gunther Sinadar: "FUEL CELLS AND THEIR APPLICATIONS", VCH Weinheim, 1996. In this publication furthermore various methods for producing membrane-electrode units for fuel cells are described. For example, the electrode can be activated by sputtering a thin platinum layer onto the diffusion layer of the gas diffusion electrode. Additional manufacturing methods are described in the German patent application with the official serial number 196 38 928.3-45. The manufacture of gas diffusion electrodes by way of a spray process is disclosed in the printed publication EP 0 687 024 A1.
The main disadvantages of the known electrode-electrolyte units with electrochemically active areas are the high costs. The high price results essentially from expensive membranes consisting for example of NAFION (a product of E.I. Dupont De Nemours) and from expensive catalysts consisting for example of platinum.
To avoid the high prices, it is being tried therefore to deposit thin catalytically active layers in electrochemically active areas. The electrochemical processes in a fuel cell occur immediately at the contact area between the gas diffusion electrode and the NAFION (a product of E.I. Dupont De Nemours) membrane. The catalyst is therefore preferably located at these contact areas, in other words, at the three-phase zone consisting of a gas distributor with electronic current conductance, the place of the electrochemical reaction and the electrolytes (in this case: NAFION (a product of E.I. Dupont De Nemours) membrane).
The printed publication U.S. Pat. No. 5,084,144 and the printed publication, E. J. Taylor, E. B. Anderson, NR K. Vilambi, Journal of the Electrochemical Society, Vol. 139 (1992) L 45-46" discloses a method for the manufacture of gas diffusion electrodes with the object to achieve a high platinum utilization for membrane fuel cells. In accordance with that method, among others, a catalyst metal is electrolytically deposited from a galvanic bath to form a thin catalytically active layer.
The disadvantage of the method disclosed in U.S. Pat. No. 5 084 144, is that it requires expensive liquid galvanic baths which must be reconditioned in a complicated and expensive manner. Furthermore, the utilization of the precious metal dissolved in the galvanic bath is very limited so that the advantages obtained by the optimized deposition are offset for example by rinsing procedures.
It is the object of the present invention to provide a cost effective manufacturing method for an electrode-electrolyte unit.